Modulation of P450CYP3A4-dependent metabolism by P-glycoprotein: Implications for P450 phenotyping

Some compounds used for phenotyping of cytochrome P450s are substrates of P -glycoprotein (pgp). It is likely that in these cases, the level of pgp mod ulates the metabolism of in vivo probes. To address this important issue, w e have analyzed the effects of pgp on CYP3A4-mediated reactions in two newl y established cell lines (3A4/HR/MDR- and 3A4/HR/MDR+), which express CYP3A 4 in the absence and presence of pgp, respectively. In cultured cells, the presence of pgp increased the apparent K-m for the 6 beta -hydroxylase acti vity of CYP3A4 toward testosterone and cortisol by a factor of 1.7 and 4, r espectively. These steroids are poor and good substrates of pgp, respective ly, and cortisol 6 beta -hydroxylase has been frequently used as an in vivo probe for CYP3A4. Interestingly, we also found that pgp modulated the inhi bition of CYP3A4-mediated metabolism by several compounds in intact cells. Although quinidine inhibited testosterone 6 beta -hydroxylase activity in m embranes or in intact cells that expressed recombinant CYP3A4 in the absenc e of pgp, low concentrations of this compound increased CYP3A4 activity in intact cells that expressed pgp. These results imply that pharmacokinetic d rug-drug interactions involving CYP3A4 can be influenced by pgp.